Terrain adaptive powered joint orthosis

ABSTRACT

A powered device augments a joint function of a human during a gait cycle using a powered actuator that supplies an augmentation torque, an impedance, or both to a joint. A controller estimates terrain slope and modulates the augmentation torque and the impedance according to a phase of the gait cycle and the estimated terrain slope to provide at least a biomimetic response. The controller may also modulate a joint equilibrium. Accordingly, the device is capable of normalizing or augmenting human biomechanical function, responsive to a wearer&#39;s activity, regardless of speed and terrain, and can be used, for example, as a knee orthosis, prosthesis, or exoskeleton.

RELATED APPLICATIONS

This application claims priority to and benefit of U.S. ProvisionalPatent Application Ser. No. 61/435,045, filed on Jan. 21, 2011, theentire content of which is hereby incorporated by reference in itsentirety.

FIELD OF THE INVENTION

This invention relates generally to powered human augmentation devices,such as lower-extremity prosthetic, orthotic, or exoskeleton apparatus,designed to emulate human biomechanics and to normalize function,components thereof, and methods for controlling the same.

BACKGROUND

Approximately 65% of service members seriously injured in Iraq andAfghanistan sustain injuries to their extremities. Many of theseindividuals experience muscle tissue loss and/or nerve injury, resultingin the loss of limb function or substantial reduction thereof. Manydevices used for the treatment of lower-extremity pathology, e.g., kneeorthoses, are passive devices. Increasingly, robotic technology isemployed in the treatment of individuals suffering from limb pathology,either for the advancement of therapy tools or as permanent assistivedevices. Upper-extremity robotic devices provide assistance and therapyfor improved reaching and manipulation and, lower-extremity roboticdevices have been developed for the enhancement of locomotor function.

Although decades of research has been conducted in the area of activepermanent assistive devices for the treatment of lower-extremitypathology, these devices are not designed to produce a biomimeticresponse, generally described in terms of joint torque, joint angle, andother related parameters as observed in a human not having substantialmuscle tissue injury and not using any device to assist in ambulation.Therefore, the robotic devices usually result in unnatural ambulationand may even cause significant discomfort to the wearer.

As such, many commercially available knee orthoses remain passive andnon-adaptive to the wearer even today. These devices typically stabilizethe knee joint medial-laterally, and limit the extent of knee flexionand extension. As such, they do not provide power or significantassistance to the user in walking, getting out of a chair, and ascendingslopes and stairs, etc.

In level-ground walking, a healthy biological knee generally behaveslike a spring during early to mid-stance, where knee torque isproportional to knee angular position. Further, during slope descent,the biological knee generally behaves like a variable damper,dissipating mechanical energy as heat to lower the body's center of masswith each step. Still further, during slope ascent, the biological kneebehaves like a torque source, applying a non-conservative propulsivetorque throughout early to mid-stance to lift the body's center of massupwards with each step.

Some common major complications of knee extensor weakness are aninability to apply: 1) damping control during slope/stair descent, 2)spring stiffness control during early to mid-stance in level-groundwalking, and 3) non-conservative propulsive torque control forslope/stair ascent and sit-to-stand maneuvers. Due to these variouscomplications, a patient with knee extensor weakness frequentlyexperiences a decrease in self-selected walking speed for level-groundand slope/stair ground surfaces, as well as an increase in walkingmetabolism while traversing these ground surfaces. Therefore, there is aneed for improved systems and methods of permanent assistive devices forthe treatment of lower-extremity pathology.

SUMMARY

In various embodiments, the present invention provides devices andmethods for operating/controlling such devices so as to assist humanswith knee extensor weakness, normalizing and/or enhancing the wearer'sself-selected walking speed and metabolic economy. This is achievedusing a type of device called Powered Knee Othosis (PKO); the PKOdevices are capable of capable of spring stiffness control, dissipativedamping control, and non-conservative torque control in both kneeflexion and extension directions, in accordance with the gait-cycle,terrain (e.g., ground slope and stairs), and walking speed. As such, thePKO devices can adaptively provide a non-conservative propulsive torqueto assist the user in walking, getting out of a chair, and ascendingslopes and stairs.

The PKO devices can also augment knee torque during late stance,particularly during slope and/or stair ascent. Thus, the PKO devices canprovide at least a biomimetic response and optionally can be used toenhance normal biomechanical response. Offering control enhancement forboth stance and swing phases, a PKO device can be used as a permanentassistive device where actuation, sensing, power, and computation areall packaged within a small, lightweight, autonomous, manufacturable,and high cycle-life package that can readily fit within a normal pantleg, and can assist humans with weak or absent quadriceps. PKO devicescan also assist humans having uninjured leg musculature in activitiessuch as carrying a heavy load over a long distance and/or increasingelevation, to enhance their strength and endurance.

In one aspect, a method for assisting a person walking on a surface witha powered human augmentation device includes a controller. The methodincludes using the controller for determining a phase of a gait cycle,and estimating within the gait cycle, a slope of the surface. The methodalso includes supplying to a joint (e.g., knee) an augmentation torque,an impedance, or both. The impedance includes a linear spring componentand a damping component. The method also includes modulating theaugmentation torque and the impedance based on the phase of the gaitcycle and the estimated slope, to provide at least a biomimeticresponse.

In some embodiments, the estimated slope is indicative of a walking modesuch that level-ground walking mode corresponds to a substantially zeroslope, downslope walking mode corresponds to a negative slope, andupslope walking mode corresponds to a positive slope. The downslopewalking mode may include descending stairs and the upslope walking modemay include ascending stairs. The joint may be a knee.

In some embodiments, the method includes estimating walking speed, andthe augmentation torque and/or the impedance may be based on theestimated walking speed. If the phase of the gait cycle is determined tobe one of early stance and mid stance and the estimated slope issubstantially zero, the impedance may be modulated such thatcontribution of the linear spring component to the modulated impedanceis greater than contribution of the damping component. If the phase ofthe gait cycle is determined to be one of early stance and mid stanceand the estimated slope is negative, however, the impedance is modulatedsuch that contribution of the damping component is increasedsubstantially compared to contribution thereof if slope is estimated tobe substantially zero. Modulating the impedance may include varying thedamping component according to the negative slope.

In some embodiments, the augmentation torque includes a non-conservativepropulsive torque. If the phase of the gait cycle is determined to beone of early stance and mid stance and the estimated slope is positive,the non-conservative propulsive torque is provided such that themodulated augmentation torque is greater than the modulated augmentationtorque applied if the slope is estimated to be substantially zero. Ifthe phase of the gait cycle is determined to be late stance, theaugmentation torque may be modulated to correspond to a reflex torquethat is related to the estimated slope.

The method may include the step of modeling a joint equilibrium as asecond-order response to a joint-position goal to be achieved prior to anext phase of the gait cycle. The modeling may be performed during aswing phase of the gait cycle. The method may also include determiningif the joint is substantially fully flexed, during a swing phase of thegait cycle. If the joint is determined to be substantially fully flexed,modulating includes adjusting both the augmentation torque and theimpedance to be substantially zero. In some embodiments, if the phase ofthe gait cycle is determined to be early swing, the augmentation torqueis modulated according to the joint-equilibrium model such that a jointequilibrium corresponds to the joint-position goal. The impedance may bemodulated according to the joint-equilibrium model such that a jointequilibrium corresponds to the joint-position goal.

In some embodiments, estimating the slope includes kinematicallyreconstructing a path of the joint (e.g., a knee) within the gait cycle.The method may also include kinematically reconstructing a path ofanother joint (e.g., an ankle) within the gait cycle, and associatingthe path of the other joint with the path of the joint to estimate theslope. The kinematic reconstruction may include computing a pose and anorigin of a co-ordinate frame associated with a link connected to atleast one of the joint and another joint proximal to the joint. The stepof computing the pose may include creating a homogeneous transformationof the co-ordinate frame. In some embodiments, the homogeneoustransformation includes a 3×1 vector defining an origin of theco-ordinate frame, and a 3×3 matrix comprising unit vectors of theco-ordinate frame. At least one point within the co-ordinate frame maycorrespond to a link connected to the joint and/or another jointproximal to the joint. The another joint may be an ankle joint and onepoint within the co-ordinate frame can be a distal end and/or a proximalend of a tibia connected to the ankle.

In some embodiments, the augmentation torque is modulated according to apositive-force feedback. The augmentation torque modulated according tothe positive-force feedback, in combination with a natural joint torquesupplied by the human, may approximate at least a normal joint torque.The positive-force feedback may include a gain and an exponent, andmodulating may include adjusting the gain or the exponent, or both,according to the estimated slope and/or walking speed. The augmentationtorque may be modulated according to a scaling factor and/or may beattenuated according to a protocol. The augmentation torque may besupplied in addition to natural joint torque supplied by the person toachieve at least a pre-determined total joint torque response.

In some embodiments, modulating includes applying a closed-loop torquecontrol at the joint. To this end, the method may include modeling thejoint torque, and determining the phase of the gait cycle based on thejoint torque model. The augmentation torque, the impedance, and a jointequilibrium may be modulated in order to achieve at least a targetwalking speed, such as a walking speed desirable to the person. Theaugmentation torque, the impedance, and a joint equilibrium may also bemodulated in order to substantially achieve a metabolic economy inaccordance with a normative reference across at least one of walkingspeed and terrain.

In another aspect, embodiments of the invention feature a powered humanaugmentation device for assisting a person walking on a surface. Thedevice includes a powered actuator for supplying to a joint anaugmentation torque and/or an impedance that includes a linear springcomponent and a damping component. The device also includes a controllerfor (i) determining a phase of a gait cycle, (ii) estimating within thegait cycle a slope of the surface, and (iii) modulating the augmentationtorque and the impedance based on the phase of the gait cycle and theestimated slope to provide at least a biomimetic response.

In some embodiments, the estimated slope is indicative of a walkingmode, such that level-ground walking mode corresponds to a substantiallyzero slope, downslope walking mode corresponds to a negative slope, andupslope walking mode corresponds to a positive slope. The downslopewalking mode may include descending stairs and the upslope walking modemay include ascending stairs. The joint may be a knee.

In some embodiments, the controller is adapted to estimate walkingspeed, and the augmentation torque, the impedance, or both may be basedon the estimated walking speed. If the controller determines the phaseof the gait cycle to be one of early stance and mid stance and theestimated slope is substantially zero, the powered actuator may beadapted to provide the modulated impedance such that contribution of thelinear spring component to the modulated impedance is greater thancontribution of the damping component. If the controller determines thephase of the gait cycle to be one of early stance and mid stance and theestimated slope is negative, the powered actuator may be adapted toprovide the modulated impedance such that contribution of the dampingcomponent is increased substantially compared to contribution thereof ifslope is estimated to be substantially zero. The controller may also beadapted to modulate the damping component according to the negativeslope.

In some embodiments, the augmentation torque includes a non-conservativepropulsive torque and, if the controller determines the phase of thegait cycle to be one of early stance and mid stance and the estimatedslope is positive, the powered actuator may be adapted to provide thenon-conservative propulsive torque such that the modulated augmentationtorque is greater than the modulated augmentation torque applied if theslope is estimated to be substantially zero. If the controllerdetermines the phase of the gait cycle to be late stance, the poweredactuator may be adapted to provide the modulated augmentation torque,such that the modulated augmentation torque corresponds to a reflextorque that is related to the estimated slope.

In some embodiments, the controller is adapted to model, during a swingphase of the gait cycle, a joint equilibrium as a second-order responseto a joint-position goal to be achieved prior to a next phase of thegait cycle. The device may include a joint angle sensor to provide ajoint angle signal to the controller. If the controller determines,based on the joint angle signal, that the joint is substantially fullyflexed, the powered actuator may adapted to adjust both the augmentationtorque and the impedance to be substantially zero, during a swing phaseof the gait cycle. If the controller determines the phase of the gaitcycle to be early swing, the augmentation torque, impedance, or both maybe modulated according to the joint-equilibrium model such that a jointequilibrium corresponds to the joint-position goal.

In some embodiments, the device includes an inertial measurement unit(IMU), and the controller may be adapted to kinematically reconstruct apath of the joint within the gait cycle based on a signal from the IMU.The controller may also be adapted to estimate the slope based on thekinematic reconstruction. The IMU may include an accelerometer and/or agyroscope. The IMU may also include a first set of sensors associatedwith the joint (e.g., a knee) and a second set of sensors associatedwith another joint (e.g., an ankle). The controller may be adapted tokinematically reconstruct a path of the other joint within the gaitcycle based on signals from the second set of sensors, and to associatethe path of the other joint with the path of the joint to estimate theslope of the terrain.

The augmentation torque may be modulated according to a positive-forcefeedback. The augmentation torque modulated according to thepositive-force feedback, in combination with a natural joint torquesupplied by the human, may approximate at least a normal joint torque.The positive-force feedback may include a gain and an exponent, andmodulating may include adjusting the gain, the exponent, or bothaccording to the estimated slope and/or walking speed. The controllermay be adapted to modulate the augmentation torque according to ascaling factor. In some embodiments, the device includes a communicationinterface for receiving a protocol, and the controller may be adapted toattenuate the augmentation torque according to the received protocol.The augmentation torque may be supplied in addition to natural jointtorque supplied by the person to achieve at least a pre-determined totaljoint torque response.

In some embodiments, the controller is adapted to apply a closed-looptorque control at the joint. The controller may be adapted to model thejoint torque, and to determine the phase of the gait cycle based on thejoint torque model. The powered actuator may include a series-elasticactuator, and the series-elastic actuator may include a transverse-fluxmotor. In some embodiments, the series-elastic actuator includes abilateral spring and a cable drive. The series-elastic actuator may alsoinclude a buckled beam and/or a unidirectional spring.

These and other objects, along with advantages and features of theembodiments of the present invention herein disclosed, will become moreapparent through reference to the following description, theaccompanying drawings, and the claims. Furthermore, it is to beunderstood that the features of the various embodiments described hereinare not mutually exclusive and can exist in various combinations andpermutations. As used herein, the term “substantially” means±10% and, insome embodiments, +5%.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1 illustrates biological knee function of an average human in thestance and swing phases of a human gait cycle during level-groundambulation;

FIG. 2 illustrates how the knee response of an average human, describedin terms of angle, moment (i.e., torque), and power, changes as afunction of terrain slope;

FIG. 3a illustrates how the knee response may become impaired when thequadriceps extensors are weakened;

FIG. 3b illustrates how the knee response of FIG. 3a can be augmented,according to one embodiment;

FIGS. 4a and 4b schematically illustrate, during early stance and latestance, respectively, the terrain-based modulation of various componentsof knee extensor torque supplied by a powered augmentation device so asto normalize the knee response, according to one embodiment;

FIG. 4c shows adjustment of various torque and impedance parametersaccording to terrain and/or walking speed, according to one embodiment;

FIG. 5 schematically depicts a powered augmentation device according toone embodiment;

FIG. 6 illustrates the operation of a state machine of a poweredaugmentation device according to one embodiment;

FIG. 7 illustrates the operation of a powered augmentation deviceimplementing the state machine of FIG. 6, according to one embodiment;

FIGS. 8a-8c schematically depict a powered augmentation device accordingto another embodiment;

FIGS. 8d and 8e illustrate closed-loop control of the poweredaugmentation device depicted in FIGS. 8a-8c , according to twoembodiments, respectively;

FIG. 9 illustrates seamless integration of a powered augmentation devicewith a leg of a human, according to one embodiment;

FIG. 10 depicts kinematic reconstruction by a controller for controllinga powered augmentation device according to one embodiment; and

FIGS. 11a and 11b depict ankle and knee paths, respectively, eachderived using measurements from an inertial measurement unit, accordingto one embodiment.

DESCRIPTION

The entire contents of each of U.S. patent application Ser. No.12/157,727 “Powered Ankle-Foot Prosthesis” filed on Jun. 12, 2008(Publication No. US2011/0257764 A1); U.S. patent application Ser. No.12/552,013 “Hybrid Terrain-Adaptive Lower-Extremity Systems” filed onSep. 1, 2009 (Publication No. US2010/0179668 A1); U.S. patentapplication Ser. No. 13/079,564 “Controlling Power in a Prosthesis orOrthosis Based on Predicted Walking Speed or Surrogate for Same” filedon Apr. 4, 2011; and U.S. patent application Ser. No. 13/079,571“Controlling Torque in a Prosthesis or Orthosis Based on a Deflection ofSeries Elastic Element” filed on Apr. 4, 2011 are incorporated herein byreference.

FIG. 1 illustrates biological knee function in the stance and swingphases of a human gait cycle during level-ground ambulation. Throughoutearly stance 102 to mid stance 104 the knee 120 typically responds as alinear spring. This form of mechanical impedance (which can take theform of a spring, inertia or damper, acting alone or in combination)serves to cushion the foot-strike impact in accordance with the gaitspeed. In late-stance 106, the knee 120 generally behaves as a torquesource in the form of a reflex to lift the lower leg 122 off the groundsurface 130 during initiation portion 108 of the swing phase. The reflexrelease may arise from a positive force feedback mechanism within thegastrocnemius muscle. In the terminal portion 110 of the swing phase,the knee 120 first brakes the swinging lower leg 122 to limit heel riseafter toe-off and then positions the lower leg 122 optimally forabsorbing energy prior to foot strike initiation in the next gait cycle.

Typically, the human gait adapts to terrain modality, e.g., ground slopeand whether the human is ascending or descending stairs, and to walkingspeed so as to maintain balance and to achieve a metabolicallyeconomical movement pattern. FIG. 2 illustrates how the knee response,described in terms of angle, moment (torque), and power, changes as afunction of terrain slope. For example, during level-ground walkingdepicted by curve 202, the biological knee behaves like a spring, whereknee torque is proportional to knee angular position, during early tomid-stance 212. During slope descent, depicted by curve 204, thebiological knee behaves like a variable damper, dissipating mechanicalenergy as heat to lower the body's center of mass with each step, duringearly to mid-stance 212. The variable damping generally increases as thedeclination angle increases. Such behavior may also be invoked duringstair descent. During slope ascent, depicted by curve 206, thebiological knee behaves like a torque source, applying anon-conservative propulsive torque throughout early to mid-stance 212 tolift the body's center of mass upwards with each step. Such behavior isusually also invoked upon stair ascent. A slope-dependent reflex isapplied in late stance 214.

Flexion angle in the swing phase also shows terrain dependence. In slopeascent, the flexion angle just prior to foot-strike, i.e., late swing222 of the curve 232 increases with the slope of ascent, whereas theknee flexion is invariant with the slope of descent, as depicted by thecurve 234. To achieve sufficient toe clearance on descent, the kneeflexion angle increases in early swing 224 as the descent becomessteeper. Though the data presented in FIG. 2 are captured at asubstantially constant gait speed, it is understood that the aboveimpedance and torque response on level ground and slopes typicallychanges with gait speed, in part, to account for changes in the bodymomentum and to deliver/absorb power accordingly.

PKO platforms 500, 800 described with reference to FIGS. 5 and 8,respectively, can discriminate terrain modality and speed within a gaitcycle (intra-cycle), and can also adapt the impedance, reflex, andposition response in accordance with that terrain and gait speed.Intra-cycle sensing is advantageous, because during an average walkterrain and walking speed may change frequently. The platforms 500, 800employ a six-degree-of-freedom inertial measurement unit (IMU) capableof computing the path of the ankle joint and the distal-end of the femur(knee), from which the IMU can discriminate and discern terrainmodality, including stairs and slopes, as illustrated with reference toFIG. 11b . The path of the hip can be used to augment the informationfrom the knee and ankle. For instance, in stair ascent, the hip isgenerally stationary as the knee flexes, a precursor that is not evidentwhen a wearer is traversing sloping and/or level ground.

FIG. 3a illustrates how the knee response may become impaired when thequadriceps extensors are weakened. In early stance 302, the kneestiffness can be insufficient to absorb energy either as a spring as inlevel-ground ambulation or as a damper in steep descent. In late stance304, the knee torque is insufficient to “brake” the knee and to deliversufficient reflex particularly in steep ascent and descent.

When worn by a wearer with weakened quadriceps extensors, the PKOplatforms 500, 800 deliver an augmentation torque, Γ_(augment), tonormalize the response, i.e., to produce a response that may be producedby a joint (e.g., knee) of average humans not having weakened muscletissue (e.g., quadriceps extensors) and not wearing any poweredprosthetic/orthotic devices. With reference to FIG. 3b , just prior tofoot-strike in early stance 312, the PKO platforms 500, 800 apply acomputed knee flexion angle and set the impedance, for energyabsorption, in accordance with terrain slope. The terrain slope can beinferred from the ankle and knee trajectories and with instantaneousgait speed inferred from the IMU-computed angular pitch rate of thefemur and tibia.

Once the foot strikes the ground in early stance 312, the PKO platforms500, 800 apply appropriate knee extensor torque, τ_(extensor), toachieve an impedance relation of the form:

τ_(extensor)=Γ₀(ϕ,{dot over (s)})−k _(ϕ,{dot over (s)})(θ−θ₀)−b_(ϕ,{dot over (s)}){dot over (θ)}

in accordance with the computed terrain slope and speed. In late stance314, the PKO platforms 500, 800 apply additional torque and reflex inaccordance with the terrain slope and the instantaneous gait speedinferred by femur and tibia pitch rates. In late stance 314, the kneeextensor torque corresponds to a biologically-conceived, non-linear,positive torque feedback relation of the form:

$_{extensor} = {P_{{{ff}\; \varphi},\overset{.}{s}}\left( \frac{\Gamma_{knee}}{\Gamma_{0}} \right)}^{N_{\varphi,\overset{.}{s}}}$

where the gain, P_(ff ϕ,{dot over (s)}) is a function of terrain slope,ϕ, and gait speed, and the exponent, N_(ϕ,{dot over (s)}), is also afunction of terrain slope and gait speed. Γ_(knee), is an intrinsicmeasure of knee torque in the above relation that includes thecontribution of both the “locking torque” of the knee and the normalizedextensor/flexor contribution. In general, both the gain and the exponentare increased to achieve the higher reflex torques needed as the slopeof ascent and descent increase.

With reference to FIG. 4a , in early stance, during level-groundwalking, the linear spring component k 402 of the extensor torqueapplied by the PKO platforms 500, 800 is significant. While descendingslope, the linear spring component k 402 is decreased and the dampingcomponent b 404 is increased, such that the damping component b 404 issignificant. While ascending slope, both the linear spring k 402 anddamping component b 404 are decreased and a non-conservative propulsivetorque component Γ₀ 406 is increased.

With reference to FIG. 4b , in late stance, during level-ground walking,the knee extensor torque applied by the PKO platforms 500, 800corresponds to non-linear, positive torque feedback determined by gain412 and exponent 414. While descending slope, the gain 412 is decreasedand the exponent 414 is increased. While ascending slope, both the gain412 and exponent 414 are increased. Adjustment of various torque andimpedance parameters according to terrain and/or walking speed isdescribed in a Table in FIG. 4c . Thus, the PKO platforms 500, 800 canemulate human knee behavior during the gait cycle by biomimeticallyapplying impedance, torque, and joint equilibrium control in accordancewith the gait cycle and speed, and augment the knee torque of the wearerto provide at least a normalized knee response.

With reference to FIG. 5, the PKO platform 500 uses a quiet,light-weight, and rugged actuator 502. A modular battery 504 having a3000 step capacity (typically for a wearer weighing about 70 kg withsignificant quadriceps extensor weakness) is used. A typical wearer mayneed to replace this lightweight battery pack 504 between one and twotimes per day. The actuator 502 can deliver at least biomimetic torqueand angle response within a gait speed range from about 0 up to about1.75 m/sec.

Optionally, the Platform 500 may employ one or two embedded wirelessinterfaces 506. A Bluetooth® interface may be used as the pathway forPDA-based tuning by clinicians and researchers to normalize the torqueresponse, e.g., by specifically programming the PKO platform 500 todeliver augmentation torque Γ_(augment) as required in each phase of thegait cycle as described below with reference to FIG. 7. A smart WiFiinterface may serve as the pathway for researchers to acquire controlstate variables and sensory feedback from the PKO platform 500 and tosynchronize this telemetry with external biomechanical instrumentation.

The actuator 502 of the PKO platform 500 can be a series-elasticactuator (SEA) to drive the powered orthosis. See, for example, U.S.Pat. No. 5,650,704 “Elastic Actuator for Precise Force Control” thedisclosure of which is incorporated herein by reference. Amulti-processor control system (State and Actuator Controller) 508 usesfeedback from the SEA to deliver the appropriate response in accordancewith the phase of the gait cycle, the terrain, and the walking speed. Athree-phase brushless motor driver (Motor Driver) 522 interfaces to theState and Actuator Controller 508 to accomplish closed-loop torquecontrol of the SEA 502. An Inertial Measurement Unit (IMU) 510,employing a three-axis rate gyro and a three-axis accelerometer,provides feedback to sense transitions between phases of the gait cycle,to measure gait speed, and to discriminate terrain modality. TheWiFi/Bluetooth® communication module 506 is employed to interfacedirectly to the State Controller and Actuator Controller 508 tofacilitate data acquisition and PDA-based clinician tuning.

The SEA 502 may employ a robust ball-screw mechanism 524 driven by thehigh-rpm brushless motor 522 through a redundant aramid fiber twin belttransmission 526. The ball-nut 524 of the SEA 502 drives the knee 540through a bilateral spring assembly 528 and a redundant aramid fibercable drive 530. The bilateral spring assembly 528 can exhibit a weakstiffness in flexion and a stiffer spring in extension as would beapplied in locking the knee joint. Thus in this embodiment, thebilateral spring 528 is used (i) to store energy in late stance forlater release in the reflex response and (ii) to serve as a sensingmeans for achieving closed-loop torque control of the actuator 502. Bystoring energy for later release, the peak power and, hence, size andweight of the motor 522 are reduced by over 40% compared to an actuatorwithout the spring storage, in this embodiment. Displacement of thespring 528 can be used to estimate and thereby control drive torque in away that attenuates the effect of friction, enabling a backdrivablemeans of actuation that replicates biological knee operation.

A knee sensor 532, a motor-position sensor 534, and a ball-screwposition sensor 536 embedded in the actuator 502 are employed todetermine a state of the actuator 502 and to provide a basis forbrushless motor control and for modulation of impedance, torque, andposition in accordance with the phase of the gait cycle and gait speed.To this end, the State Controller and Actuator Controller 508 implementsa state machine.

With reference to FIG. 6, during early stance state 602, the statemachine 600 adapts the PKO platform 500 to apply a linear spring anddamping impedance in accordance with the gait speed and terrain angle,given by:

τ_(extensor)=Γ₀(ϕ,{dot over (s)})−k _(ϕ,{dot over (s)})(θ−θ₀)−b_(ϕ,{dot over (s)}){dot over (θ)}

k _(cp) =k _(cp)(ϕ,{dot over (s)});b _(cp) =b _(cp)(ϕ,{dot over (s)})

Where

T_(extensor) is the commanded SEA motor torque

θ is the ankle angle,

ϕ is the terrain angle, and

{dot over (s)} is the estimated gait speed at foot-strike estimated bythe IMU

Transition into the early stance state 602 is accomplished by sensing bythe IMU 510 the distinctive vibration that occurs when the foot strikesthe ground. The impedance is configured and scaled so as to preventbuckling of the knee in accordance with walking speed and the responseneeded to at least normalize the augmented response of the wearer.

Transition into the late stance state 604 generally occurs when thedetected knee extension angle velocity changes from negative topositive. In this state 604, a reflex response can be achieved throughnon-linear positive feedback as described by the relation:

$_{extensor} = {P_{{{ff}\; \varphi},\overset{.}{s}}\left( \frac{\Gamma_{knee}}{\Gamma_{0}} \right)}^{N_{\varphi,\overset{.}{s}}}$

In this, the reflex gain, P_(ff)(ϕ,{dot over (s)}) and the exponent(non-linear spring), N(ϕ,{dot over (s)}) are each a function of theterrain angle, ϕ, and the estimated gait speed, {dot over (s)}={dot over(s)}({dot over (Ψ)}_(femur),{dot over (Ψ)}_(tibia)), which is a functionof the instantaneous angular rate of the tibia and femur at the time ofentry in to the late stance state 604. A hard stop spring model forextreme knee extension, Γ_(knee) (θ), is used to model the wearer torqueresponse at extremes of extension (θ>0) while the knee is locked so thatat least a biomimetic response is achieved.

Transition into early swing state 606 occurs when the detected SEA 502torque, Γ_(SEA), approaches a programmable percentage of peak torque. Inthis state 606, position control is employed to brake the knee flexionvelocity, to achieve proper ground clearance and heel rise during theearly to mid swing phase through use of an organically-derivedtrajectory, θ₀(t) that smoothly decelerates to a goal position in anearly ballistic trajectory (i.e., small torque corresponding to alightly damped pendulum), θ_(goal)=θ_(goal) ₀ =θ_(goal) ₀ ({circumflexover (ϕ)}|_(ls), {dot over (s)}):

τ_(extensor) =−k _(esw)(θ−θ₀)−b _(esw)({dot over (β)}_(motor)−{dot over(β)}_(motor) ₀ )

τ_(trajectory) ²{umlaut over (θ)}₀+2τ_(trajectory){dot over(θ)}₀+θ₀=θ_(goal) ₀

θ_(goal) ₀ =θ_(goal) ₀ ({circumflex over (ϕ)}|_(ls) ,{dot over(s)});{dot over (β)}=J ⁻¹(θ){dot over (θ)}

where β_(motor) is the motor angle corresponding to a knee angle withzero SEA spring displacement. and

{circumflex over (ϕ)}|_(ls) is estimated terrain angle as estimated atthe end of late stance using the inertial tibia and femur angularvelocities.

Also in the early swing state 606, the inertial ankle and kneetrajectories are computed and used to discriminate between the threemodalities, i.e., slope/stair ascent, slope/stair descent, and walkingon substantially level ground. This early discrimination may be used toadjust the control parameters of the State Controller and ActuatorController 508 in advance of foot strike to achieve seamless responseacross the swing-stance transition.

Transition into late swing state 608 occurs when the IMU 510 detects anegative, vertical Cartesian (world-frame referenced) ankle pivotvelocity, ^(W)V_(ankle pivot) _(z) . In this state 608, position controlis used with a smooth trajectory that converges to a time-varying goalpoint, θ_(goal), that is a function of gait speed and terrain slope,each estimated by the IMU 510 which in some embodiments uses onlyintra-gait-cycle information. The impedance (stiffness and damping)applied to position and velocity errors referenced to the trajectory(equilibrium), θ₀(t) may be preferably set in accordance with gait speedand terrain angle.

FIG. 7 illustrates how the PKO platform 500 can augment the torque of awearer to achieve at least a normalized biomimetic response. In someembodiments, a powered augmentation device can augment the torque andadjust impedance to achieve a response that can enable a wearer who doesnot have a diminished natural joint function to perform activities suchas walking or running a long distance, carrying a heavy load, climbingsteep slopes, etc. The state machine 600 modulates the SEA 502impedance, reflex, and position control response in accordance with gaitspeed and terrain modality inputs from the IMU 510. The SEA 502 controlinternally computes at least the normalized biomimetic torque, Γ*, ineach state of the gait cycle. State-specific attenuation, set by theclinician, then scales Γ* and drives the SEA 502 to deliver just theright torque, Γ_(augment), to add to the wearer's natural torqueresponse, Γ_(wearer), to approximate Γ*, i.e., the desired normalizedbiomimetic response or an enhanced response that may allow a person toundertake activities such as walking fast (e.g., 2 m/sec.) for a longtime e.g., about 6 hours.

Battery conservation is important in wearable PKO devices. In theabsence of battery energy, or when the walking state machine (e.g., thestate machine 600, illustrated with reference to FIG. 6) detects thatthe wearer has stopped walking (which can be determined by absence ofgait-cycle phase transition for over approximately two seconds), thecontrol system shorts the motor leads to ground using power electronics.In this special damping mode the motor leads are shorted together,creating a dynamic brake with damping torque,τ_(motor)=−b_(sl)ω=−(k_(g)k_(t))²/Rω, where b_(sl) is the shorted leadsdamping, k_(g) is the gear ratio between the motor and joint output,k_(t) is the motor constant in Nm/A and R is the motor resistance, and ωis the rotation rate of the joint. In the “shorted leads” operation, thetime constant, τ_(sl), that describes the first-order spring-damperactuator dynamics comprising the series-spring, k_(SEA) and theintrinsic actuator damping, b_(sl), is given by the relation,

$_{sl} = {\frac{b_{sl}}{k_{SEA}}.}$

In transverse-flux and other high-torque motor actuators, the τ_(sl) maybe on the order of about 500 msec or more. For time intervals, e.g.,less than ⅓ of the time constant, the actuator 502 in “shorted leads”mimics a static clutch, taking no energy from the battery. By matchingthe series-stiffness with that required in early stance flexion, themotor clutch is engaged at the desired joint equilibrium so as toapproximate the biomimetic linear spring response without requiring anybattery energy. This affords significant advantage in system design,response, and economy of operation.

FIGS. 8a-8c depict a PKO device 800 that employs a buckled beam 812 asthe series-elastic element of the SEA 802. The SEA 802 includes a highRPM brushless, permanent magnet motor 814 having an integral heat sinkand an insulator. The motor 814 can be a radial motor, a transverse-fluxmotor, a stepping motor, etc. The SEA 802 also includes a sealedball-screw mechanism 816 having a 14 mm diameter and 3 mm lead, in thisembodiment. It should be understood that these dimensions areillustrative only and are not limiting.

The motor 814 is coupled to the buckled beam via a flexural coupling 818to protect the ball-screw mechanism 816 from moment load, a reverse-camlinkage 820, and sealed needle bearings 824. The needle bearings 824typically have L1 design life of over five million cycles (i.e., adesign whereby 99% of a population survive longer than the reporteddesign life with 95% statistical confidence). The PKO 800 also includesan integral pivot scaffold SEA support 826, coupled to the motor 814,and a foot support 828 (e.g., a custom nylon foot support), coupled tothe buckled beam 812. The reverse-cam linkage 820 includes an encoder830 that may be used to determine the SEA torque based on atorque-angular displacement model. The encoder 830 can be a 13-bitabsolute encoder having a torque resolution of about 8 bits.

In one embodiment, the motor 814 is controlled in a closed loop. FIG. 8dillustrates one embodiment of an implementation of the closed-looptorque control in the PKO 800, in which the Joint Torque CommandGenerator 852 computes the commanded joint torque, Γ_(joint), fromterrain, ϕ, walking speed, {dot over (s)}, and gait-cycle phase as theseare supplied from a State Controller (e.g., State and ActuatorController 508, described with reference to FIG. 5). The Joint-TorqueModel 854 estimates the actual applied joint torque, Γ_(joint), fromwearer knee extension, wearer extensor-flexor and buckling-beam 812 (forseries-elasticity) torque contributions. The wearer contributions may beassumed to be a percentage of a normative amount or a percentage of thecommand torque. The contribution of the buckling-beam 812 (serieselastic component of the SEA 802, in general) may be estimated fromoff-wearer calibration during testing of the PKO device 800.

The difference in the commanded and applied torque, δΓ_(joint), isscaled by the nominal stiffness of the buckling beam 812 (generally, theSEA) and is passed through a proportional-integral-derivative (PID)compensator 856, G₁(z⁻¹), to compute a commanded value of deflection,β−θ, where θ is the joint angle and β is the joint angle specified bythe actuator for approximately zero buckled beam (SEA) deflection. G₁ isdesigned with at least integral compensation with saturation errorlimits to force substantially zero steady-state torque error and maytypically include proportional and derivative terms. The sensed jointangle, θ_(sense), is added by an adder 858 to the deflection command tocompute a commanded actuator angle, β_(commanded).

The estimated actuator displacement is derived by actuator kinematics860 by sensing the motor angle, p, which is used in a computationalmodel, β(p), of the actuator kinematics 860. The actuator error issupplied to a second PID compensator 862 with actuator range of motionlimits to deliver a motor torque, τ_(motor), to drive the actuator 802.A brushless, permanent magnet motor, either radial, transverse flux, orstepping motor, is commutated electronically using a multiphase motordriver that delivers a torque-producing current component, i_(q), toachieve the desired motor torque via the relation τ_(motor)=k_(t)i_(q),where k_(t) is the motor torque constant in Nm/A. If a stepping motor isused, the motor can be stepped in a closed-loop fashion to align withthe position command.

In another embodiment illustrated with reference to FIG. 8e , the JointTorque Model 854 supplies and estimated joint torque to the Joint TorqueCommand generator 852, which determines the augmentation torque command,Γ_(joint). The torque command is passed through a command shaping filter864, having a transfer function G₁ (z⁻¹) and a torque de-scaling,1/k_(SEA), to create a high-fidelity deflection signal. The commandshaping filter 864 may be a low-pass filter to ensure that the innerdeflection control loop has sufficient response bandwidth to follow thecommand. Other embodiments may be implemented by those skilled in theart to deliver a joint torque response that closely matches the desiredbiomechanical response as this is achieved through modulation ofimpedance, joint equilibrium, and torque in accordance with gait-cyclephase, terrain and walking speed.

Seamless integration of the PKO platform 500 onto a wearer is desirableto ensure that the torque supplied by the PKO platform 500 is coupledefficiently to the joint (knee, ankle, etc.). With reference to FIG. 9,in some embodiments, a process is provided for custom manufacturing anupper cuff assembly 902 and a lower cuff assembly 904 to conform/coupledirectly to the wearer. For each wearer a three-dimensional scanningtool is employed to measure those body surfaces that must integrate withthe PKO platform 500. From these surface measurements, lightweighttitanium forms can be printed (e.g., using a direct-write process).These can be functionalized through heat treating to create the scaffoldupon which a custom 3-D printed elastomer, with spatially-varyingdurometer, can be bonded to achieve the desired custom integration.

In some embodiments, the State and Actuator Controller 508 is adapted tokinematically reconstruct a joint path. Such reconstruction can be usedto determine the terrain (e.g., whether the terrain is level ground,sloping ground, or stairs), and activity (i.e., whether the wearer iswalking on level ground, upslope, or downslope, or walking up or downthe stairs). The modulation of the toque, impedance, and jointequilibrium may be based on the terrain and activity as determined viathe kinematic reconstruction.

FIG. 10 illustrates a method for determining, via kinematicreconstruction, ankle joint 1000, heel 1012 and toe 1016 paths whileusing any PKO device (e.g., the PKO platforms 500, 800) based on theinertial pose of a lower leg member 1020 coupled to the ankle joint1000, and the angle between the lower leg member 1020 and foot member1008. Pose is the position and orientation of a coordinate system. TheIMU (e.g., the IMU 510) may be coupled to the lower leg member 1020. TheIMU may include a three-axis rate gyro for measuring angular rate and athree-axis accelerometer for measuring acceleration. Placing theinertial measurement unit on the lower leg member 1020 collocates themeasurement of angular rate and acceleration for all three axes of thelower leg member 1020. The inertial measurement unit provides asix-degree-of-freedom estimate of the lower leg member 1020 pose,inertial (world frame referenced) orientation and ankle-joint 1000(center of rotation of the ankle-foot) location.

In some embodiments, the lower leg member 1020 pose is used to computethe instantaneous location of the knee joint. By using knowledge of theankle joint 1000 angle (θ) the instantaneous pose of the bottom of thefoot 1008 can be computed, including location of the heel 1012 and toe1016. This information in turn can be used when the foot member 1008 isflat to measure the terrain angle in the plane defined by the rotationalaxis of the ankle joint/foot member. Mounting the inertial measurementunit on the lower leg member 1020 has advantages over other potentiallocations. Unlike if it were mounted on the foot member 1008, the lowerleg member 1020 mounting protects against physical abuse and keeps itaway from water exposure. Further, it eliminates the cable tether thatwould otherwise be needed if it were on the foot member 1008—therebyensuring mechanical and electrical integrity. Finally, the lower legmember 1020 is centrally located within the kinematic chain of a hybridsystem facilitating the computation of the thigh and torso pose with aminimum of additional sensors.

The inertial measurement unit can be used to calculate the orientation,_(ankle) ^(w)O, position, _(ankle) ^(w)p, and velocity, _(ankle) ^(w)v,of the PKO platform in a ground-referenced world frame. _(ankle) ^(w) Omay be represented by a quaternion or by a 3×3 matrix of unit vectorsthat define the orientation of the x, y and z axes of the ankle joint inrelation to the world frame. The ankle joint 1000 coordinate frame isdefined to be positioned at the center of the ankle joint axis ofrotation with its orientation tied to the lower leg member 1020. Fromthis central point, the position, velocity and acceleration can becomputed. For points of interest in, for example, the foot (e.g., theheel 1012 or toe 1016), a foot member-to-ankle joint orientationtransformation, _(foot) ^(ankle)O(θ) is used to derive the positionusing the following relation:

_(point-of-interest) ^(w) p= _(ankle) ^(w) p+ _(ankle) ^(w) O(γ)_(foot)^(ankle) O(θ)(^(foot) r _(point-of-interest))

where

$\;_{foot}^{ankle}{O(\gamma)} = \begin{bmatrix}1 & 0 & 0 \\0 & {\cos (\gamma)} & {- {\sin (\gamma)}} \\0 & {\sin (\gamma)} & {\cos (\gamma)}\end{bmatrix}$

where γ is the inertial lower leg member angle, and

$\;_{foot}^{ankle}{O(\theta)} = \begin{bmatrix}1 & 0 & 0 \\0 & {\cos (\theta)} & {- {\sin (\theta)}} \\0 & {\sin (\theta)} & {\cos (\theta)}\end{bmatrix}$

where θ is the ankle joint angle.

In this embodiment, the inertial measurement unit, including thethree-axis accelerometer and three-axis rate gyro, is located on theforward face at the top of the lower leg member 1020. It is advantageousto remove the effect of scale, drift and cross-coupling on theworld-frame orientation, velocity and position estimates introduced bynumerical integrations of the accelerometer and rate gyro signals

Inertial navigation systems typically employ a zero-velocity update(ZVUP) periodically by averaging over an extended period of time,usually seconds to minutes. This placement of the inertial measurementunit is almost never stationary in the lower-extremity devices such as aPKO. However, the bottom of the foot is the only stationary location,and then only during the controlled dorsiflexion state of the gaitcycle. An exemplary zero-velocity update method, which is not impactedby this limitation, for use with various embodiments of the invention isdescribed further below.

To solve this problem, orientation, velocity and position integration ofankle joint is performed. After digitizing the inertial measurement unitacceleration, ^(IMU)a, the ankle joint acceleration (^(IMU) a_(ankle))is derived with the following rigid body dynamic equation:

^(IMU) a _(ankle)=^(IMU) a+ ^(IMU){right arrow over (ω)}X ^(IMU) {rightarrow over (ω)}X _(ankle) ^(IMU) {right arrow over (r)}+{right arrowover ({dot over (ω)})}X _(ankle) ^(IMU) {right arrow over (r)},

where ^(IMU) {right arrow over (ω)} and ^(IMU){dot over ({right arrowover (ω)})} are the vectors of angular rate and angular acceleration,respectively, in the inertial measurement unit frame and X denotes thecross-product.

The relationship is solved _(ankle) ^(w)O=_(IMU) ^(w)O similarly as inthe equations above using standard strapdown inertial measurement unitintegration methods, in accordance with the following relationshipsknown to one skilled in the art:

$\;_{ankle}^{w}{\hat{\Phi} = {{{{\,^{w}\hat{\Omega}}\left( {\,^{w}\hat{\omega}} \right)}_{ankle}^{w}\hat{\Phi}{\, {{}_{}^{}\left. v \right.\hat{}_{}^{}}}} = {{{}_{}^{}\left. a \right.\hat{}_{}^{}} - \left\lbrack {0,0,g} \right\rbrack^{T}}}}$${{}_{}^{}\left. p \right.\hat{}_{}^{}} = {{}_{}^{}\left. v \right.\hat{}_{}^{}}$ _(foot)^(w)Φ̂=_(ankle)^(w)Φ̂ _(foot)^(ankle)Φ̂ =  _(ankle)^(w)Φ̂Rotation_(x)(Θ̂)${{}_{}^{}\left. v \right.\hat{}_{}^{}} = {{{}_{}^{}\left. v \right.\hat{}_{}^{}} + {\left. {\,^{w}\hat{\Omega}}(_{ankle}^{w}{\hat{\Phi}\begin{bmatrix}\hat{\overset{.}{\Theta}} & 0 & 0\end{bmatrix}}^{T} \right){{}_{}^{}{}_{{heel} - {ankle}}^{}}}}$${{}_{}^{}\left. v \right.\hat{}_{}^{}} = {{{}_{}^{}\left. v \right.\hat{}_{}^{}} + {\left. {\,^{w}\hat{\Omega}}(_{ankle}^{w}{\hat{\Phi}\begin{bmatrix}\hat{\overset{.}{\Theta}} & 0 & 0\end{bmatrix}}^{T} \right){{}_{}^{}{}_{{toe} - {ankle}}^{}}}}$${{}_{}^{}\left. p \right.\hat{}_{}^{}} = {{{}_{}^{}\left. p \right.\hat{}_{}^{}} + {{}_{}^{}{}_{{heel} - {ankle}}^{}}}$${{}_{}^{}\left. p \right.\hat{}_{}^{}} = {{{}_{}^{}\left. p \right.\hat{}_{}^{}} + {{}_{}^{}{}_{{toe} - {ankle}}^{}}}$${{}_{}^{}{}_{{heel} - {ankle}}^{}} = {{{}_{}^{}\left. \Phi \right.\hat{}_{}^{}}\left( {r_{heel} - r_{ankle}} \right)}$${{}_{}^{}{}_{{toe} - {ankle}}^{}} = {{{}_{}^{}\left. \Phi \right.\hat{}_{}^{}}\left( {r_{toe} - r_{ankle}} \right)}$

In the equations above, the matrix, {circumflex over (Φ)}, will be usedinterchangeably with the orientation matrix, _(IMU) ^(w)O. The worldframe-referenced ankle joint velocity and position are then derived at apoint in time after the time of the previous zero-velocity update (i-thzero-velocity update) based on the following:

^(w) v _(ankle)(t)=∫_(ZVUP(i)) ^(t)(_(IMU) ^(w) O)^(IMU) a _(ankle) dt

^(w) p _(ankle)(t)=∫_(ZVUP(i)) ^(t w) v _(ankle) dt

where ^(w)p_(ankle)(t=ZVUP(i)) is reset to zero for all i.

The six-degree-of-freedom inertial measurement unit (IMU) 510 of the PKOplatform 500 or the IMU of the PKO device 800 is capable of computingthe path of the ankle joint and the distal-end of the femur (knee) fromwhich the IMU can discriminate and discern terrain modality—includingstairs and slopes. With reference to FIG. 11a , inertially referencedankle joint paths 1102, ^(W)p_(ankle joint)(t), andankle-velocity-attack-angle 1104, ^(W)V_(ankle joint), on stairs andsloping ground can be used to discriminate stair ascent/descent fromascent/descent on sloping ground. The slope, ϕ, can be estimated as{circumflex over (ϕ)} in swing using the relation:

{circumflex over (ϕ)}=tan⁻¹(^(w) p _(ankle joint) _(z) (t),^(W) p_(ankle joint) _(y) )

With reference to FIG. 11b , inertially-referenced knee path shape canbe used to detect stair ascent/descent shortly after toe-off—enablingknee impedance and torque response to be configured prior to foot-strikeon the stair. The “kink” 1110 in the knee path may signal impending footstrike on sloping ground, enabling a prediction of terrain slope usingthe ankle joint slope prediction described above with reference to FIG.11a . Using the joint slope, speed and ankle velocity angle-of-attack,the joint equilibrium and impedance can be adjusted in preparation forthe foot strike.

While the invention has been particularly shown and described withreference to specific embodiments, it will be understood by thoseskilled in the art that various changes in form and detail may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims. The scope of the invention is thusindicated by the appended claims and all changes that come within themeaning and range of equivalency of the claims are therefore intended tobe embraced.

What is claimed is:
 1. A method for assisting a person walking on a surface with a powered human augmentation device including a controller, the method comprising the steps of: determining a phase of a gait cycle; estimating within the gait cycle, a slope of the surface; supplying to a joint at least one of an augmentation torque and an impedance comprising a linear spring component and a damping component; and modulating the augmentation torque and the impedance based on the phase of the gait cycle and the estimated slope to provide at least a biomimetic response.
 2. The method of claim 1, wherein the estimated slope is indicative of a walking mode such that level-ground walking mode corresponds to a substantially zero slope, downslope walking mode corresponds to a negative slope, and upslope walking mode corresponds to a positive slope.
 3. The method of claim 2, wherein the downslope walking mode comprises descending stairs and the upslope walking mode comprises ascending stairs.
 4. The method of claim 1, wherein the joint is a knee.
 5. The method of claim 1 further comprising estimating walking speed, wherein at least one of the augmentation torque and the impedance is based on the estimated walking speed.
 6. The method of claim 1, wherein if the phase of the gait cycle is determined to be one of early stance and mid stance and the estimated slope is substantially zero, the impedance is modulated such that contribution of the linear spring component to the modulated impedance is greater than contribution of the damping component.
 7. The method of claim 1, wherein if the phase of the gait cycle is determined to be one of early stance and mid stance and the estimated slope is negative, the impedance is modulated such that contribution of the damping component is increased substantially compared to contribution thereof if slope is estimated to be substantially zero.
 8. The method of claim 7, wherein modulating comprises varying the damping component according to the negative slope.
 9. The method of claim 1, wherein the augmentation torque comprises a non-conservative propulsive torque; and if the phase of the gait cycle is determined to be one of early stance and mid stance and the estimated slope is positive, providing the non-conservative propulsive torque such that the modulated augmentation torque is greater than the modulated augmentation torque applied if the slope is estimated to be substantially zero.
 10. The method of claim 1, wherein if the phase of the gait cycle is determined to be late stance, the augmentation torque is modulated to correspond to a reflex torque that is related to the estimated slope.
 11. The method of claim 1 further comprising the step of modeling, during a swing phase of the gait cycle, a joint equilibrium as a second-order response to a joint-position goal to be achieved prior to a next phase of the gait cycle.
 12. The method of claim 11 further comprising the step of determining if the joint is substantially fully flexed, during a swing phase of the gait cycle, wherein modulating comprises adjusting both the augmentation torque and the impedance to be substantially zero, if the joint is determined to be substantially fully flexed.
 13. The method of claim 11, wherein if the phase of the gait cycle is determined to be early swing, the augmentation torque is modulated according to the joint-equilibrium model such that a joint equilibrium corresponds to the joint-position goal.
 14. The method of claim 11, wherein if the phase of the gait cycle is determined to be early swing, the impedance is modulated according to the joint-equilibrium model such that a joint equilibrium corresponds to the joint-position goal.
 15. The method of claim 1, wherein estimating the slope comprises kinematically reconstructing a path of the joint within the gait cycle.
 16. The method of claim 15 further comprising the steps of: kinematically reconstructing a path of another joint within the gait cycle; and associating the path of the other joint with the path of the joint to estimate the slope.
 17. The method of claim 15, wherein the kinematic reconstruction comprises computing a pose and an origin of a co-ordinate frame associated with a link connected to at least one of the joint and another joint proximal to the joint.
 18. The method of claim 17, wherein computing the pose comprises creating a homogeneous transformation of the co-ordinate frame.
 19. The method of claim 18, wherein the homogeneous transformation comprises: a 3×1 vector defining an origin of the co-ordinate frame; and a 3×3 matrix comprising unit vectors of the co-ordinate frame.
 20. The method of claim 17, wherein at least one point within the co-ordinate frame corresponds to a link connected to at least one of the joint and another joint proximal to the joint.
 21. The method of claim 20, wherein the another joint is an ankle joint and the at least one point is at least one of a distal end and a proximal end of a tibia connected to the ankle.
 22. The method of claim 1, wherein the augmentation torque is modulated according to a positive-force feedback.
 23. The method of claim 22, wherein the augmentation torque modulated according to the positive-force feedback, in combination with a natural joint torque supplied by the human, approximates at least a normal joint torque.
 24. The method of claim 22, wherein the positive-force feedback comprises a gain and an exponent.
 25. The method of claim 24, wherein modulating comprises adjusting at least one of the gain and the exponent according to at least one of the estimated slope and walking speed.
 26. The method of claim 1, wherein the augmentation torque is modulated according to a scaling factor.
 27. The method of claim 1 further comprising the step of attenuating the augmentation torque to be applied according to a protocol.
 28. The method of claim 1, wherein the augmentation torque is supplied in addition to natural joint torque supplied by the person to achieve at least a pre-determined total joint torque response.
 29. The method of claim 1, wherein modulating comprises applying a closed-loop torque control at the joint.
 30. The method of claim 29 further comprising: modeling the joint torque; and determining the phase of the gait cycle based on the joint torque model.
 31. The method of claim 1, wherein the augmentation torque, the impedance, and a joint equilibrium are modulated in order to achieve at least a target walking speed.
 32. The method of claim 1, wherein the augmentation torque, the impedance, and a joint equilibrium are modulated in order to substantially achieve a metabolic economy in accordance with a normative reference across at least one of walking speed and terrain. 33-62. (canceled) 